1. Field of the Invention
The present invention generally relates to a method and apparatus for polishing metal layers deposited on wafers, and more particularly relates to a method and apparatus for pulse electrochemical polishing for fabricating semiconductor devices.
2. The Related Art
As known to all, a deposition process is an absolutely necessary process for fabricating a semiconductor device, which is performed to deposit a metal layer in trenches and vias and also on non-recessed areas of a wafer. To isolate interconnections formed on the wafer, the metal layer deposited on the non-recessed areas of the wafer needs to be removed. Generally, the metal layer is deposited on a dielectric layer of the wafer. Accordingly, the metal layer deposited on non-recessed areas of the dielectric layer needs to be removed.
To remove the metal layer deposited on the non-recessed areas of the dielectric layer, a chemical mechanical polishing (CMP) method is used and includes steps of: providing a rotatable table, a polishing pad disposed on the rotatable table, a wafer carrier head for gripping the wafer and a slurry feeder providing slurry between the wafer and the polishing pad; applying a downward press force on the wafer carrier head to press the wafer against the polishing pad, which enforces the wafer to rotate relatively to the polishing pad. Then, the wafer is polished.
However, for continually shrinking the feature dimension of the semiconductor devices, Cu and low-k dielectic or ultra low-k dielectric are used in the semiconductor device fabrication. Because the Cu and the low-k dielectic or the ultra low-k dielectric have very weak mechanical stress, so the downward press force acted on the wafer carrier head in the CMP process will damage the Cu and the low-k dielectic or the ultra low-k dielectric.
To solve the above problem, an electrochemical polishing method is employed to remove the metal layer on the non-recessed areas of the dielectric layer. Firstly, provide a wafer chuck for holding the wafer thereon and a nozzle arranged in a polishing chamber for ejecting electrolyte on the metal layer of the wafer. The wafer chuck can linearly move in a horizontal direction and rotate during the electrochemical polishing process. A power supply electrically connects with the wafer chuck and the nozzle. The wafer chuck electrically connects the anode of the power supply and the wafer and the nozzle electrically connects the cathode of the power supply and charges the electrolyte. The electrochemical polishing method utilizes the chemical reaction between the electrolyte and the metal layer to polish the metal layer. In the polishing process, only the electrolyte contacts with the metal layer, so the metal layer can be removed without mechanical stress, solving the Cu and the low-k dielectic or the ultra low-k dielectric integration issue.
Generally, the motion speed of the wafer chuck and the electric current/voltage provided by the power supply dominate the removal thickness of the metal layer. The electric current/voltage set value will be changed according to the metal layer thickness on the non-recessed areas of the dielectric layer. If a wafer surface topography is not uniform, for example, there are humps and valleys on a post-plating pattern wafer, due to the metal layer thickness is huge difference in adjacent regions and the wafer rotates at a high speed during the polishing process, the electric current/voltage should be changed rapidly and discretely in a very short time to match the wafer topography thickness change. Actually, it is very hard to make the electric current/voltage be changed rapidly and discretely in a very short time. As a result, the method is only suitable for polishing a blanket wafer but can't accurately polish the post-plating pattern wafer.